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Original question:

AMC3302: Zero crossing distortion in AMC3302

AMC3302: AMC3302 and ISO224 Cross over distortion

Benjamin Maxwell
Prodigy 50 points
Part Number: AMC3302
Other Parts Discussed in Thread: ISO224, ,

Tool/software:

Hello,

I am experimenting with the AMC3302 (and ISO224) for isolated current sensing in a power electronics control application. We notice that at low frequencies (<10Hz) there is a significant distortion that appears to be 0 crossing distortion present in the amplified signal. I was wondering if 1. This is expected in these isolated amplifiers 2. Is there something that can be done to minimize the distortion in these units.

Here is a captured waveform with the input (green) and output (yellow) for the ISO224 where the distortion is visible.

Thanks.

  • 0
    TI__Expert 3480 points

    Hi Benjamin,

    Did you see this distortion with the AMC3302 also?  

    From the picture it looks like the input is 6mVpp, is that correct?

    Are you measuring this on the EVM?  If not, could you provide a circuit schematic?

    Is the distortion you are referring to the ~1mV blip on the output near when the input crosses 0?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 50 points

    Hey Saleh,

    We see a similar distortion on the AMC3302.

    Inputs are 6mV pk-pk. 

    This is a custom board but we also see this on the EVM.

    We can change the location of the distortion in the waveform by changing the offset leading us to belive it's 0 crossing distortion. Adding enough of an offset also causes the distortion to disappear. 

    I highlighted the distortion below.



  • 0 in reply to Benjamin Maxwell
    Prodigy 50 points

    Hey Saleh,

    Have you had a chance to look into this?

    Thanks.

  • 0 in reply to Benjamin Maxwell
    TI__Expert 3480 points

    Hi Benjamin,

    I tried recreating the issue with a AMC3302EVM I have, but am unable to see the distortion.  When you measured the AMC3302 distortion was the input voltage still ~6mVpp?  What was the output distortion level that you saw with the 3302?  Also, does the magnitude of the distortion increase with increased input?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 50 points

    Hey Saleh, 

    For the AMC3302EVM it was tested so the output voltage had the same amplitude or about 100uV on the input. For this we used a 1mOhm current sense resistor and used a function generator to give us 100mA signal.

  • 0 in reply to Benjamin Maxwell
    TI__Expert 3480 points

    Hi Benjamin,

    Is there a reason you are inputting such a small signal into the devices?  For best performance and resolution, you would want to scale your input up to the full scale range of the device.

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 50 points

    Hey Saleh,

    We are using this to measure amplitude noise on top of a larger DC signal. This signal is sufficiently above the specified broad band noise floor on these parts for our application. If you have some better alternatives that do not have this cross-over distortion we are open to trying something else.

    Thanks.

  • 0 in reply to Benjamin Maxwell
    TI__Expert 3480 points

    Hi Benjamin,

    Would you be able to add an amplifier gain stage before the device?  Also, could you send me some more details on the application?  What is the voltage of the DC signal you are measuring, and how are you sending the noise signal to the device?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 50 points

    We can add a gain stage. In which case we are considering using a dedicated ADC and isolating post digitization. This is more complex but would certainly not have zero crossing distortion.

    The DC voltage is full scale of the amplifier. We are selecting resistor values for to ensure this.

    The full signal with noise signal is digitized through an ADC.

    Thanks.